1. Field of the Invention
The present invention relates to a method for producing iron oxide powder, characterized in that the particle size and shape of iron oxide powder can be controlled by reacting .alpha.-FeOOH under an appropriate temperature and pressure using an organic solvent and the controller for particle size and shape. More particularly, the invention relates to a method of producing iron oxide powder in which the particle size and shape of crystals can be controlled by growing crystalline particles of iron oxide stably through a chelating reaction using glycol of a dihydric alcohol and reacting the grown crystalline particles at a relatively low temperature.
2. Description of the Prior Art
Recently, there has been increasing interest in the study of the synthesis of ceramics powder and its commercial application. The particle size and shape of ceramics powders are important factor in determining ceramics' physical properties. Thus, many researchers have been conducting studies to develop methods for controlling the particle size and shape of ceramics. Ceramics is a general term for inorganic materials. A compound consisting of Fe and O among ceramics is called as an iron oxide, and a crystal of Fe.sub.3 O.sub.4 among the iron oxide is classed as ferrite.
Namely, ferrite is an inorganic compound which has iron oxide (Fe.sub.3 O.sub.4) as a major component. Ferrite exhibits a ferromagnetic property and has been widely used in the electronics industry. Recently, interest in ferrite has also increased in the somatoscience industry because it can selectively separate and isolate certain components of an organism and thus has a potentiality for application in the development of drugs. It has been known that the magnetic property of iron oxide powder, a major component of ferrite, varies depending on the particle size and shape of iron oxide.
Ferrite particles being used at present are produced by an emulsion method that entails high production costs. Thus, there has been the search for a cheaper method of producing ferrite particles. Furthermore, there is a need to develop a method of producing ferrite powder that has large crystallinity, high affinity to organic compounds and excellent magnetic characteristics.
Generally, in methods using a solid state, it is almost impossible to prepare ceramic particulates having a particle size of not more than 1 .mu.m and there are defects that a composition of ceramics powder produced may be different from that of raw materials. Furthermore, in the solid state method, it is not easy to control the shape of the powder. To solve this problem, preparation methods using a liquid state, for example, co-precipitation, hydrothermal synthesis, sol-gel method, etc., have been actively studied with positive results. Among these methods, hydrothermal synthesis, which uses an appropriate temperature and pressure, is regarded as being able to control the composition, particle size and shape of the powder relatively easily. A lot of studies have already been made and some of them have been applied to commercial use.
At present, the studies on hydrothermal synthesis have focussed on using an organic metal compound, as a starting material, instead of a salt or a hydroxide, or on using an organic solvent rather than an aqueous solvent. For example, according to the research of R. S. Sapiesko & E. Matijevi, C., .alpha.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 could be obtained by reacting a ferric salt with a chelating agent such as TEA or EDTA under a basic atmosphere and then decomposing the resulting Fe chelate by adding an oxidizing agent and a reducing agent under an hydrothermal synthetic condition of 250.degree. C.
In addition, some studies have reported a novel method of producing ferrite powder, which can produce uniform powder at a lower temperature and pressure than that of hydrothermal synthesis through forming a complex of a metal ion with an organic solvent or its derivatives that primarily uses an organic solvent without using separate chelating agent or a metal chelate. This method is widely known as the Pechini method. According to this method, a complex of a metal ion and an organic solvent and its derivatives are formed and then the complex polymer is burned under an elevated pressure to prepare powder. It is reported that fine powder can be obtained by the method but it is not easy to control size and shape of the obtained powder.
When considering cases using an organic solvent, several ceramics powders, such as alumina, were typically produced by using mainly an alcohol or a mixed solution with its aqueous solution. Furthermore, the production of a glycol derivative and its complex through the reaction of an Al ion and a glycol was also identified. Another reported method is said to be able to prepare monodispersion .alpha.-Al.sub.2 O.sub.3 particles at a temperature of 300.degree. C. by using 1,4-butylene glycol as a solvent and which can stably control diverse shapes of the particles. When an aqueous solvent is used, the monodispersion .alpha.-Al.sub.2 O.sub.3 particles can only be prepared under a temperature of more than 500.degree. C. However, few cases have been found where ferrite powder was produced by using an organic solvent. Also, the reported method uses salts of nitric and sulfuric acid as starting materials. Unlike this method, the present invention uses a hydroxide crystal as a starting material, and as a result, it can easily control the particle size and shape of the powder.
Based on the above research results, the present invention uses a dihydric alcohol, i.e., glycol, instead of an aqueous solution to stably grow crystalline particles of iron oxide through a chelating reaction and to lower the crystallization temperature so as to control the particle size and shape of the crystals. Many studies on hydrothermal synthesis for producing iron oxides have been reported. However, a study that uses glycol as a solvent to produce iron oxides has not been reported yet.
Thus, the invention aims to control the particle size and shape of iron oxides by a glycothermal reaction which uses glycol as a solvent. The inventors of the present invention have studied the effects of propylene glycol and ethylene glycol as a solvent and several additive upon the particle size and shape of the powder and established appropriate conditions based on these studies.
3. Description of the Invention